"The Australian Taxation Office is one of our largest and most important customers," Optus Business managing director John Paitaridis said in a statement.

"The extension of Optus' contract demonstrates our proven ability to deliver managed ICT services to the scale, reliability and quality that the ATO and other leading Australian organisations require of their strategic partners."

The ATO originally signed an agreement for Optus to provide managed network services to the agency in 2009.